Some of the pathways of utilization of blood glucose and its incorporation into tissue glycogen in Limulus polyphemus have been studied. Preferential loss of C-1 from glucose in vivo prior to incorporation into glycogen indicates a significant occurrence of the oxidative pathway of glucose metabolism in Limulus. Glucose is more rapidly and extensively inmcorporated into glycogen of muscle than into that of hepatopancreas, indicative of the relative importance of muscle in metabolic turnover in Limulus. Little gluconeogenesis from lactate or py ruvate was observed. Although glucose occurs at levels of only 3-5 mg per 100 ml of blood in Limulus, it is rapidly metabolized, about 80% of the 14C of blood occurring as anions within 1 hour after administration of glucose-14C. As much as 70% of tritium administered on C-6 of glucose is excreted as H20 within 3 hours, showing an extensive, rapid glycolytic degradation of glucose in Limulus.